DESCRIPTION: Mechanistic and structural studies of stress responsive gene regulation by the MerR and Fur families of transcription factors are described in this proposal. Proteins in these families of stress responsive transcription factors control microbial responses to phagocytosis (SoxR), nutrient starvation during infection (Fur), and antibiotic treatments (MerR, BrmR and TipAL). In a variety of virulent microorganisms, Fur, or a closely related iron-sensor protein, controls toxin expression. In the latter case, a general but controversial mechanism for iron-responsive derepression has been proposed. The primary goal of this proposal is to characterize the mechanism of transcriptional activation by the mercury-specific metalloregulatory protein, MerR. In parallel studies the mechanism of repression by the iron-responsive meralloregulatory protein, Fur, will be probed. These studies focus on energetic and stereochemical aspects of biopolymer conformation changes in the allosteric switching mechanism. This proposal plans to characterize in both systems the molecular basis of metal ion recognition of these metal receptors. MerR controls expression of bacterial mercurial responsive genes in novel manner, that is, the mechanism for positive control in this allosteric system is mediated by protein-induced distortions in DNA structure. Positive control mechanisms are poorly understood and yet are of fundamental importance in understanding the molecular basis of genetic regulation. Therefore, this proposal seeks to determine the molecular basis of heavy metal recognition in both the MerR and Fur system at the biopolymer and coordination chemistry levels. Special emphasis will be placed on developing solution (199)Hg NMR methods as probes of coordination environments in these and other metalloproteins. These molecular studies which focus on microbial metal ion sensory mechanisms are suggested to provide general models that will serve as a starting point for understanding the cell biology of metals in general.